Laser-based apparatus for refraction-correction ophthalmologic surgery, such as the LASIK (laser in situ keratomileusis) procedure, can correct various vision impairments such as myopia (i.e., near-sightedness), hyperopia (i.e., farsightedness) and astigmatism by surgically reshaping the cornea of the eye. Hyperopia is measured in terms of positive diopters. Myopia is measured in terms of negative diopters. The most common refractive errors ranged between +6 to −6 diopters. For example, if part of the corneal stroma (the interior bulk of the cornea) is removed, the created void can be made to close. The result is a reshaped cornea.
Conventionally, LASIK procedures use a mechanical knife, called a keratome, to create a flap, The mechanical flap-creation procedure can damage the cornea and stroma tissue, possibly requiring an extended healing period and leaving undesirable artifacts such as haze, scarring, and/or instability of the correction, which interfere with vision in some cases.
Further, once the flap is created and folded back, conventional LASIK procedures typically use a high-powered ultraviolet excimer laser to photoablate a pattern (e.g. of spots) to reshape the stroma. Such a process is relatively crude, due to the relatively large size of the spots and possibly due to heating and acoustic shockwaves from the excimer photoablation. That is, the relatively course granularity of the excimer laser procedure and its higher energy pulses leaves something to be desired. After shaping the stroma, the flap is repositioned to complete the surgery.
As described in U.S. Pat. No. 6,110,166 issued Aug. 29, 2000 entitled “Method for corneal laser surgery” (and incorporated herein by reference), a LASIK-type surgery procedure can be made more effective and efficient if the flap that is created can be repositioned in an interlocking relationship with the undisturbed corneal tissue. A flap with an interlockable configuration can be created. The flap could then be lifted to expose the corneal tissue that is to be removed and, next, after the desired amount of corneal tissue is removed, the flap could be repositioned and interlocked with undisturbed corneal tissue to hold the flap in place during the healing process. The use of laser systems for ophthalmic surgical procedures, such as for other procedures contemplated for the present invention, is particularly appropriate due to the extreme precision required when corneal tissue is to be removed. Depending on the diameter and the general shape of the tissue volume to be removed, the removal of a layer of stromal tissue that is only approximately ten microns thick can result in a one diopter change. The removal of a lens shaped volume of tissue that is four millimeters in diameter and approximately fifty microns thick at its center can result in a active correction of approximately four diopters Thus, for vision corrections to achieve accuracy within one-diopter, the surgical procedure employed must be capable of precisely removing corneal tissue having a thickness which is accurate to within less than ten microns. Further, this degree of accuracy applies for any refractive correction regardless of the total amount of correction required.
The correction of myopia requires removal of a volume of conical tissue having a different shape than does the correction of hyperopia. Also, the limits of potential correction are different. For a myopic correction, a lentoid or lens-shaped volume of stromal tissue is removed. At the present time, myopic corrections of up to approximately thirty diopters can be reasonably expected. On the other hand, corrections of hyperopic conditions can be made up to only about fifteen diopters. Furthermore, for a hyperopic correction the volume of stromal tissue that is removed is thicker towards the edges than in the center.
Conventional femtosecond laser apparatus for eye surgery takes a relatively long time to form the cuts, typically in the order of one minute, during which time the eye must be held in a fixed position in order that the cuts are contiguous and formed in the shape that was predefined.
There is thus a need for an improved apparatus for fast refraction-correction ophthalmologic surgery in order to change the corneal curvature in a controlled way without affecting the corneal clarity or the integrity of the various membranes surrounding the stroma.